Curved retarder-based optical filters

ABSTRACT

Curved polarization filters and methods of manufacturing such filters. The method includes laminating a planar polarization layer to a planar retarder layer at a predetermined orientation and bending the laminate to create a curved filter. The strain on the retarder layer results in stress-induced birefringence, and the predetermined orientation of the retarder substantially compensates for the stress-induced birefringence. In some embodiments, the predetermination is based on mathematical models. In some other embodiment, the predetermination is based on experimental data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims priority under 35 U.S.C. 119(e) to U.S.Provisional Patent Application No. 60/979,326, entitled “Method andApparatus for Curved Retarder-based Optical Polarization Filters” filedOct. 11, 2007.

TECHNICAL FIELD

This disclosure generally relates to optical filters, and moreparticularly, this disclosure relates to curved optical filters forviewing stereoscopic or non-stereoscopic images.

BACKGROUND

Stereoscopic imaging involves recording three-dimensional (3-D) visualinformation or creating the illusion of depth in an image. One easy wayto create depth perception in the brain is to provide the eyes of theviewer two different images, representing two perspectives of the sameobject, with a minor deviation similar to the perspectives that botheyes naturally receive in binocular vision. Many optical systems displaystereoscopic images using this method. The illusion of depth can becreated in a photograph, movie, video game, or other two-dimensional(2-D) image.

BRIEF SUMMARY

Stereoscopic and non-stereoscopic eyewear may include a low-cost opticalfilter manufactured by laminating a retarder film (e.g., a quarter waveplate (“QWP”)) and a polarizer film from separate roll stock. Laminatinga retarder film from roll stock to a polarizer film from roll stockinvolves cutting, aligning, and laminating the films at a suitableorientation that allows for the desired optical property. The end resultof this process is a planar optical filter.

Disclosed in the present application is an optical filter in a curvedconfiguration. The optical filter includes a polarization layer and aretarder layer laminated to the polarization layer at a predeterminedangle, wherein the retarder and polarization layers are bent to thecurved configuration. The retarder layer has a stress-inducedbirefringence, and the predetermined angle substantially compensates forthe stress-induced birefringence and disposes the optical axis of theretarder film in a desired orientation. In one embodiment, the retarderlayer is a QWP. In another embodiment, the retarder layer is amulti-layer retarder stack. In another embodiment, the polarizer layeris disposed on the inner layer of the retarder layer and an additionalnon-birefringent layer is laminated to the outer surface of the retarderlayer. Other configurations are possible.

The present disclosure also provides a method for manufacturing a curvedfilter having a retardation axis in a desired orientation. The methodincludes providing a planar retarder film having an optical axis and aplanar polarization film. The method also includes predetermining anangle at which the planar retarder film would be laminated to the planarpolarization film, and laminating the planar retarder film and theplanar polarization film at the predetermined angle. The method furtherincludes bending the laminated retarder and polarization films, wherebya stress is exerted on the retarder film, the stress causing astress-induced birefringence in the retarder film. The predeterminedangle substantially compensates for the stress-induced birefringence andallow the optical axis of the retarder film to be in the desiredorientation.

The present disclosure further provides an apparatus for laminatingcurved optical films, which includes a curved drum operable to provide acurved surface for bonding first and second optical films. The apparatusalso includes at least two feeders operable to feed the first and secondoptical films onto the curve drum, a coating apparatus operable to applyadhesive to a surface of the first or second optical film, and a pressroller operable to apply pressure against the first and second opticalfilms and laminate the first and second optical films. Methods ofmanufacturing a filter using such an apparatus are also provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) is a perspective schematic view of a planar polarizer filter;

FIG. 1(B) is a perspective schematic view of a curved polarizer filter;

FIG. 1(C) is a perspective schematic view of a curved polarizer filterhaving an optical axis in a desired orientation;

FIG. 2 is a chart correlating cylindrical curve radius with retarderangle adjustment for a curved filter;

FIG. 3 is a chart illustrating the optical performance of variousfilters;

FIG. 4 is a perspective schematic view of a curved polarizer filterhaving a compound curvature;

FIG. 5 is a perspective schematic view of another embodiment of a curvedpolarizer filter;

FIG. 6 is a schematic diagram of an embodiment of a laminationapparatus; and

FIG. 7 is a display system incorporating curved filters in eyewear forviewing stereoscopic images.

DETAILED DESCRIPTION

One technique to provide 3-D images is to encode light bound for eacheye with different polarizations. Such a scheme may involve usingorthogonal linearly polarized states, or circularly polarized states ofopposite handedness. To present a stereoscopic picture using acircularly polarized 3-D image system, two images are projected andsuperimposed onto the same screen through circular polarizing filters ofopposite handedness. The viewer wears eyeglasses that include a pair ofcircular polarizers (“CPs”) of opposite handedness, which function asanalyzers. Light that is of left-circular polarization is blocked by theright-handed analyzer while right-circularly polarized light is blockedby the left-handed analyzer. The result is similar to that ofstereoscopic viewing with linearly polarized glasses, except the viewercan tilt his or her head and not compromise the quality of thepolarization encoding and decoding.

Commonly-assigned U.S. Pat. No. 4,792,850 by Lenny Lipton, which ishereby incorporated by reference, discloses electronically driven CPsthat alternate between left and right handedness in synchronization withthe left and right images being displayed by the image projector. Directview displays may also be used to encode the polarization states fordifferent eyes; for example, alternate pixels of a direct view displaymay provide light of different polarization states. Another way toprovide alternate right/left eye images is using a single display orprojector that actively encodes the images using a polarization switch.Examples of such a technique are disclosed in the commonly-assigned U.S.patent application Ser. No. 11/424,087, entitled “AchromaticPolarization Switches,” filed Jun. 14, 2006, which his herebyincorporated by reference.

Instead of encoding the 3-D information with polarization, the left andright eye images may be encoded on distinct wavelength bands asdisclosed by Maximus et al. in U.S. patent application Ser. No.10/576,707. Eyewear is then used to selectively filter the wavelengthsappropriate for each eye. As described by Maximus et al., this filteringmay be performed using dichroic filters; however, it is also possible toperform this filtering using polarization interference.

Additionally, the selective filtering of light by polarizationinterference may be used to enhance vision and/or protect eyes fromharmful light rays. For example, such filtering may be used insunglasses, color corrective eyewear, or protective eyewear. Theselective filtering of incident light may provide any desired spectraltransmission (including visible light and light not visible to the eye).The filtering structure may include multi-layer polarizing structuresand may be formed by fabricating sheet laminates that are dye-cut toform inexpensive laminates. One embodiment of the apparatus operable toprovide selective filtering includes a pair of polarizing elements thatsandwich a retarder stack. Further details of the design of such eyewearare described by Sharp in commonly-assigned U.S. Pat. No. 7,106,509,which is hereby incorporated by reference.

In order to watch a motion picture using polarization encoding, theviewer wears a pair of paper frame or plastic frame glasses withpolarization filters. The filters in such glasses are generally producedby laminating a retarder film to a polarizing film encapsulated bytriactyl cellulose (“TAC”) using adhesives. In some embodiments, theretarder film may be a QWP and the adhesive used for lamination is apressure sensitive adhesive (“PSA”). In other embodiments, the retarderfilm may be a half-wave plate (“HWP”). Such viewing eyewear uses aplanar filter.

A retarder, as described in the present application, may comprise Nlinear retarders that have been designed using Finite Impulse Response(FIR) filter techniques, wherein the impulse response of the N retardersgenerates at least N+1 output impulses from a polarized impulse input.As such, placing retarder stacks between neutral polarizers forms FIRfilters, and these FIR filters can be designed using standard signalprocessing methods. The amplitude of each responsive output impulse isdetermined by the orientations of the retarders and the analyzingpolarizer. Further details of the design approaches for the describedretarder stacks are described in the commonly assigned U.S. Pat. No.7,106,509 and by U.S. patent application Ser. No. 09/754,091, which isalso incorporated by reference herein.

In general, the component stock films for retarders and polarizers haveplanar geometry. However, it is desirable both for optical improvementand for cosmetic enhancement to produce eyewear with curved filters.Shaping the filter to form a structure with either cylindrical orcompound curvature subsequent to the lamination of component stock filmscan induce unacceptable strain in the component films. This straineither induces birefringence or modifies the desired intrinsicbirefringence and thus degrades the performance of the filter.

In U.S. Pat. No. 5,051,309, Kawaki et al. disclose a method of formingpolarizing glasses employing linear polarization elements. Kawaki'smethod involves either annealing the birefringent polycarbonate layersthat are in front of the polarizing element to eliminate thebirefringence or by “super-stretching” these layers to have a very largeretardance parallel to the polarizer axis. The polarizer effectivelyhides the birefringence of the polymer. In U.S. Pat. No. 6,177,032,Smith et al. disclose a method of pre-forming the individual functionallayers and then assembling the final filter. Again, birefringence of thelayers preceding the polarizer is altered by annealing.

When a specific retardation and retarder angle are required, such aswhen CPs are used in combination with retarder stack filters, neither ofthese methods are acceptable. Thus, there remains a need for curvedfilters incorporating retarder-based polarization filters, in which thestress-induced birefringence is either compensated or minimized.

Exemplary embodiments of a curved optical filter and methods ofmanufacturing such a filter will be discussed below with references toFIGS. 1 to 6. Specifically, various methods and apparatus to compensatefor or minimize strain-induced changes in the retarder layer of a filterwill be disclosed.

FIG. 1(A) shows a planar filter 100 having a retarder layer 104laminated to a polarizer 106. The retarder layer 104 has an optical axis112 along axis 120 and the polarizer layer 106 has an optical axis 114along axis 122. In the illustrated embodiment, the retarder layer 104 isa QWP, but in other embodiments, the retarder layer 104 may be ahalf-wave plate (“HWP”) or any retarders described in the presentapplication. In some embodiments additional optical components may beadded to the planar filter 100. For example, planar filter 100 mayfurther include a substrate layer to provide better structural support.In some other embodiments, the ordering of the retarder and polarizerlayers 104 and 106 may be inverted. It is to be appreciated that thestructure of the filter 100 may be varied to satisfy various designneeds.

In the embodiment in FIG. 1(B), the filter 100 is bent around a firstbend axis (not shown) to form a curved filter 101 by pulling on ends 102of the filter 100. The cylindrical filter 101 can be mounted on aglasses frame to retain the desired curved configuration. It is to beappreciated that, in other embodiments, the filter 101 can be bentaround multiple bent axes to have a compound curvature. In someembodiments, thermoforming processes known in the art may be used toretain the desired curved configuration. In one embodiment, the filter101 is bent around first and second bend axes, and undergoes athermoforming process to retain a compound curvature.

Because of the finite thickness of the filter 100, a compressive stress110 is exerted on the polarizer layer 106 while a tensile stress 108 isexerted on the retarder layer 104. In both layers, the directions ofstresses 108 and 110 are tangent to the curvature of the cylindricalfilter, and the magnitude of each stress is uniform across the surfaceof the curved filter 101.

The optical effect of stretching or bending a material that displaysstrain birefringence can be conveniently described using the straintensor,

. The eigenvectors (

₁,

₂,

₃) of

are parallel to the principle axes of the dielectric tensor (oftenerroneously called optic axes). The birefringence, Δn, is proportionalto the difference in the magnitudes of the eigenvalues of

, and the retardance is equal to the integral of Δn across the thicknessof the film; Δn·d, in the case of uniform films of thickness d. Formaterials exhibiting linear elasticity, sequential stretching operationsare equivalent to adding the strain tensor for each individualoperation:

=

+

.  (1)

The strain tensor of a stretched polymer retarder film with retardance Γand slow axis oriented parallel to the x-axis can be written:

$\begin{matrix}{{{\overset{\leftrightarrow}{ɛ}}^{\prime} = {\frac{1}{t \cdot K}\begin{bmatrix}\Gamma & 0 & 0 \\0 & 0 & 0 \\0 & 0 & 0\end{bmatrix}}},} & (2)\end{matrix}$where K is the strain optic coefficient and d is the film thickness. Forconvenience we neglect strain in the orthogonal directions, i.e., wechoose a Poisson ratio of 0 although it can be shown that the derivationis general. It is trivially seen that additional stretching parallel tothe x-axis increases the retardance while stretching perpendicular tothe x-axis decreases the retardance. However, if the stretching isperformed at a 45 degree angle to the x-axis, the additional straintensor is:

$\begin{matrix}{{\overset{\leftrightarrow}{ɛ}}^{''} = {{\frac{1}{2}\begin{bmatrix}ɛ^{''} & {- ɛ^{''}} & 0 \\{- ɛ^{''}} & ɛ^{''} & 0 \\0 & 0 & 0\end{bmatrix}}.}} & (3)\end{matrix}$

The eigenvalues of

are then:

$\begin{matrix}\begin{matrix}{ɛ_{1} = {\frac{1}{2}\left( {\frac{\Gamma}{d \cdot K} + ɛ^{''} - \sqrt{\left( \frac{\Gamma}{d \cdot K} \right)^{2} + \left( ɛ^{''} \right)^{2}}} \right)}} \\{ɛ_{2} = {\frac{1}{2}\left( {\frac{\Gamma}{d \cdot K} + ɛ^{''} + \sqrt{\left( \frac{\Gamma}{d \cdot K} \right)^{2} + \left( ɛ^{''} \right)^{2}}} \right)}}\end{matrix} & (4)\end{matrix}$and the orientation of

is

$\begin{matrix}{\theta = {- {{\tan^{- 1}\left( {2\frac{\frac{\Gamma}{d \cdot K} - \sqrt{\left( \frac{\Gamma}{d \cdot K} \right)^{2} + \left( \frac{ɛ^{''}}{2} \right)^{2}}}{ɛ^{''}}} \right)}.}}} & (5)\end{matrix}$

When equation 5 is expanded for ∈″>>Γ/(d·K) it can be shown that forsmall additional stretching at 45 degrees from the optic axis, thechange in the retarder orientation is linear in ∈″. In contrast,equation 4 can be solved for An to show that the change in retardationis quadratic in ∈″ in the same limit.

Based on the above discussion, it is apparent that stretching parallelor perpendicular to the retarder optic axis only changes the magnitudeof the retardance while the optic axis orientation is stable. Incontrast, stretching at 45 degrees to the optic axis rotates the opticaxis for relatively small strains.

Referring back to FIG. 1(B), the polarizer axis 122 is parallel to thestrain direction, and accordingly, its orientation remains stable. Thetensile stress 108, however causes an in-plane rotation of the retarderaxis 112 by θ (equation 5) as well as an increase in the retardanceaccording to equation 4. The retarder axis 112 is now oriented alongaxis 124. Due to the in-plane rotation of the retarder axis 112, thefilter 101 now produces elliptical instead of circularly polarizedlight, and the discrimination between left and right eye images iscompromised.

Referring now to FIG. 1(C), when the stretching is uniform in bothmagnitude and direction across the surface of the film, it is possibleto predict the change in both retardance and optic axis orientation andto compensate for it by adjusting the initial retardance and orientationof the film. Therefore, even materials such as polycarbonate with alarge strain-optic coefficient may be used. In the embodimentillustrated in FIG. 1( c), the retarder and polarizer layers arelaminated at a predetermined angle to compensate for the stress-inducedbirefringence in the retarder film and to dispose the retarder axis inthe desired orientation as indicated by axis 120.

In order to produce retardance Γ parallel to the x-axis in the presenceof uniform strain of magnitude ∈ with principle strain directionoriented at θ degrees to the x-axis, it can be shown that an initialretardance ofΓ₀=√{square root over (Γ² +K ² d ²∈²−2K·d·∈·Γ cos 2θ)},  (6)should be oriented at:

$\begin{matrix}{\theta_{0} = {- {{\tan^{- 1}\left( \frac{\left( {{- \Gamma} + {{K \cdot d \cdot ɛ \cdot \cos}\; 2\;\theta} + \sqrt{\begin{matrix}{\Gamma^{2} + {K^{2}d^{2}ɛ^{2}} -} \\{2\;{K \cdot d \cdot ɛ \cdot \Gamma}\;\cos\; 2\;\theta}\end{matrix}}} \right)\csc\; 2\theta}{K \cdot d \cdot ɛ} \right)}.}}} & (7)\end{matrix}$

Equations (6) and (7) are applied individually for each layer in amulti-layer stack to account for differences in strain, retardance, andretardance orientation. The trivial solution occurs when the desiredretardance is zero. Equations (6) and (7) reduce to the intuitive resultthat the initial retardance is chosen to exactly cancel the retardanceinduced by strain.

These conclusions can also be extended to polarizing components. Thepolymer matrix of a polarizer film may be highly stretched such that theassumptions about additive strain from the previous section are unlikelyto apply. However, the symmetry remains the same: the polarization axisorientation will remain stable under parallel (and perpendicular)stretching, but may be rotated if the stretching occurs at 45 degrees tothe polarizer axis. Consequently it is desirable to orient the polarizeraxis parallel or perpendicular to the principle component of anyexpected strain.

It is possible to either analytically or numerically model themechanical structure of the cylindrical filter 101 that would allow thetensile stress 108 in the outer retarder layer 104 to balance thecompressive stress 110 on the inner polarization layer 106. By knowingthe elastic constants for the layers of the filter 101, it is thenpossible to solve for the tensile stress 108 and then use equations (6)and (7) to compute and predetermine the initial retardance and retarderorientation that would compensate for the stress-induced birefringence.By laminating the retarder and polarizer layers at a predeterminedoff-set angle 128, the optic axis 112 of the retarder layer 104 would berotated from axis 126 to the desired axis 120 as illustrated in FIG.1(C) due to the bending of the curved filter 101 and the stress-inducedbirefringence.

In practice, it is often more convenient to simply measure the change inthe retarder film at different radii of curvature and then generate alook-up table or chart for the off-set angle that would compensate forthe stress birefringence. Due to the quadratic correction to theretardance for small strain, the same retardation film may be used for abroad range of curvatures while only adjusting the film orientationduring lamination.

An exemplary lookup chart for a CP filter is shown in FIG. 2. Plottedalong the x-axis are the various radii of the CP filter and plottedalong the y-axis are the corresponding off-set angles that wouldcompensate for the stress-induced birefringence. The data were obtainedby measuring the ellipticity of light exiting CP filters with varyingradii of curvature. The analytic expression for the optic axisorientation was then used to determine the optic axis orientation in thebent filter and thus the offset angle for compensating it. For largeradius curves, the offset angle goes to zero. For very small radiuscurves, the relation eventually breaks down as changes in retardancebecome increasingly important. It is to be appreciated that similarlookup charts can be created for other types for filters in accordanceto the principles disclosed herein.

FIG. 3 is a chart illustrating the optical performance of variousfilters. The improved performance of a filter that has been adjusted tocompensate for stress-induced birefringence relative to that of a filterthat has not been pre-adjusted is demonstrated in FIG. 3. FIG. 3 shows aset of spectra obtained for two different filters that were crossed witha reference filter. The first filter is manufactured with a 45 degreeoptic axis orientation (without pre-adjustment) for the retarder; thesecond filter was pre-adjusted for an 8 cm radius of curvature using thecutting angle offset in FIG. 2. In the planar configuration, the firstfilter produced leakage below 0.2% over much of the visible spectrumwhereas the second filter had much higher leakage as expected. When bentto a radius of curvature of 8 cm, the first filter produced leakagegreater than 0.4% over the entire spectrum—highlighting the need forcorrection. The second filter performed nearly as well with a radius of8 cm as the first filter did in the planar configuration.

In addition to adjusting the initial retardance and retarderorientation, stress-induced birefringence can also be reduced by usingretarder films with optimal material properties. Irrespective of thestretching direction, the effect of induced strain decreases as themagnitude of the initial retardance increases. Therefore, it isadvantageous to use as large of an initial retardance as possiblerelative to the quantity K·d·∈″, in order to minimize the effect ofinduced strain on the final structure. Accordingly, it is desirable tominimize the thickness, d, of the film and/or the strain opticcoefficient, K. Specifically, it is preferred that the ratio

$\frac{K \cdot d \cdot ɛ^{''}}{\Gamma}$is less than 0.01 in order to maintain approximately a 1% uniformity inthe retardance and optic axis orientation.

Satisfying the above prescription for any significant strain isdifficult with polycarbonate-based retardation films because of therelatively high strain-optic coefficient of polycarbonate. When usingsuch films, it is therefore preferable to choose thinner film stock. Inthe case in which the polycarbonate retarder film is approximately 60 μmthick, decreasing the thickness to 12 μm only increases the strain-opticstability by a factor of 5. Such thin films may be difficult to workwith in the manufacturing setting and may be susceptible to damageduring any solvent welding or other lamination processes. Improvedstrain resistance can be obtained by using optical plastics such ascellulose di-acetate or cyclic olefin copolymer (“COC”) retarder film.The strain-optic coefficient of these materials is approximately 10times smaller than that of polycarbonate, and thus, a thicker substratemay be used to ease manufacturability while still minimizingstress-induced birefringence. An exemplary embodiment is the COC-basedfilm manufactured under the brand name Arton®. This material hasexcellent optical clarity, uniform birefringence, and a sufficientlysmall strain optic coefficient to enable thermoforming in a variety ofapplications.

FIG. 4 is a schematic view of a curved filter 200 that was bent around aplurality of bend axes and configured to have a compound curvature. Thefilter 200 includes a retarder layer 202 laminated to a polarizer layer204. The filter 200 further includes a substrate layer 206. The compoundcurvature may be retained using a thermoforming process. In oneembodiment, the thermoforming process includes applying heat to thesubstrate layer. To minimize the effect of stress birefringence resultedfrom the bending of the filter 200, the retarder layer 202 is preferablymade of materials having strain optic coefficient ranging from 0.001 to0.025. The thickness of the retarder layer is preferably between 50 to120 microns. In an exemplary embodiment, the retarder layer 202 is madeof cyclic olefin block copolymer. It is to be appreciated that theretarder layer can be made of other materials and have variousthicknesses so long that the ratio

$\frac{K \cdot d \cdot ɛ^{''}}{\Gamma}$is approximately less than 0.01.

FIG. 5 is a perspective schematic view of another embodiment of a curvedpolarizer filter 300. Curved filter 300 includes a polarizer layer 314having optic axis 306, a retarder layer 316 having an optic axis 304,and a non-birefringent mechanical layer 318. The mechanical layer 318 isadded to sandwich the retarder layer 316 to balance the stain on theretarder layer 316. In some embodiments, the mechanical layer 318 mayinclude a negative c-plate. The thickness and elastic modulus of themechanical layer 318 is preferably chosen so that it exerts a tensilestress 312 that balances compressive stress 310, which would minimizethe stress-induced birefringence in the retarder layer 316. Furthermore,the strain optic coefficient of the mechanical layer 318 preferably issufficiently small that the tensile stress 312 imparts minimaladditional in-plane birefringence. In an exemplary embodiment, thepolarizer layer 314 may include a TAC encapsulated polarizer, and itwould be sufficiently balanced mechanically by an equal thickness of TACin the mechanical layer 318. As result, the orientation of the opticaxis 304 of the retarder layer 316 would not be affected bystress-induced birefringence.

FIG. 6 is a schematic diagram of an embodiment of a lamination apparatus400 for manufacturing a curved filter while minimizing the strain on thecomponents of the curved filter. The lamination apparatus 400 includes acurved drum 406 operable to provide a curved surface for bonding firstand second optical films 402 and 404. It is preferable that the radiusof the drum 406 matches the radius of the finished eyewear. In anexemplary embodiment, the first optical film 402 is a polarizer film,and the second optical film 404 is a retarder film, such as a QWP film.In another exemplary embodiment, the second film 404 is a QWP orientedat 45 degrees to the first film 402. The lamination apparatus 400further includes at least two feeders 412 operable to feed the first andsecond optical films onto the curved drum 406 and a coating apparatus414 operable to apply adhesive to a surface of the first or secondoptical film. A press roller 410 of the lamination apparatus 400 isoperable to apply pressure against the first and second optical films402 and 404 to laminate them.

In operations, to laminate the first and second films 402 and 404, eachfilm is fed onto the drum at different rates to accommodate thedifferent radius of curvature of each film. The films 402 and 404 travelon the drum surface for a finite time through region 408 in order toallow the internal stress to be relieved. Press roller 410 completes thelamination process by exerting pressure on the films 402 and 404 andcausing the adhesive to form bonds between the films 402 and 404.

It is to be appreciated that the curved filters disclosed in the presentapplication can be incorporated into various eyewear for viewingstereoscopic or non-stereoscopic images displayed by any imaging systemsdescribed herein. For example, a display system 600 as illustrated inFIG. 7 can include a projection screen 602 and polarization filteringeyewear 604 that incorporates two curved filters 606 and 608.Stereoscopic 3-D imagery is observed using a singlepolarization-preserving screen 602 sequentially displaying left andright perspective imagery, with polarization filtering eyewears 604. Insome embodiments, the curved filters 606 and 608 are of alternatelyorthogonal polarization. In some particular embodiments, the curvedfilters 606 and 608 are operable to provide circularly polarized lightof opposite handedness. In an exemplary embodiment, thepolarization-preserving screen 602 is a direct-view screen.

It will be appreciated by those of ordinary skill in the art that theinvention(s) can be embodied in other specific forms without departingfrom the spirit or essential character thereof. Any disclosed embodimentmay be combined with one or several of the other embodiments shownand/or described. This is also possible for one or more features of theembodiments. The steps herein described and claimed do not need to beexecuted in the given order. The steps can be carried out, at least to acertain extent, in any other order.

Further, it will be appreciated by one of ordinary skill in the art thatvarious retardance and optic axis values depend sensitively on themechanical properties of all of the layers and adhesives in the filter.It will also be appreciated that the circular polarizer disclosed hereinmay be combined with various other display components to perform similarresults. The presently disclosed embodiments are therefore considered inall respects to be illustrative and not restrictive. The scope of theinvention is indicated by the appended claims rather than the foregoingdescription, and all changes that come within the meaning and ranges ofequivalents thereof are intended to be embraced therein.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 C.F.R. §1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theinvention(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called technical field.Further, a description of a technology in the “Background” is not to beconstrued as an admission that technology is prior art to anyinvention(s) in this disclosure. Neither is the “Brief Summary” to beconsidered as a characterization of the invention(s) set forth in theclaims found herein. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty claimed in this disclosure. Multipleinventions may be set forth according to the limitations of the multipleclaims associated with this disclosure, and the claims accordinglydefine the invention(s), and their equivalents, that are protectedthereby. In all instances, the scope of the claims shall be consideredon their own merits in light of the specification, but should not beconstrained by the headings set forth herein.

What is claimed is:
 1. A method for manufacturing a curved filter havinga retardation axis in a desired orientation, the method comprising:providing a planar retarder film having an optical axis; providing aplanar polarization film; predetermining an angle at which the planarretarder film would be laminated to the planar polarization film;laminating the planar retarder film and the planar polarization film atthe predetermined angle; bending the laminated retarder and polarizationfilms, whereby a stress is exerted on the retarder film, the stresscausing a stress-induced birefringence in the retarder film; and whereinthe predetermined angle substantially compensates for the stress-inducedbirefringence and allow the optical axis of the retarder film to be inthe desired orientation.
 2. The method of claim 1, wherein thepredetermined angle is determined using a mathematical model.
 3. Themethod of claim 1, wherein the predetermined angle is determined using achart or table, the chart or table providing a correlation between theradius of the curved filter to the predetermined angle.
 4. The method ofclaim 1, wherein the planar retarder film comprises a quarter-waveplate.
 5. The method of claim 1, wherein the planar retarder filmcomprises a multi-layer retarder stack.
 6. The method of claim 1,wherein the planar retarder film is made of cyclic olefin blockcopolymer or cellulose di-acetate.
 7. The method of claim 1, furthercomprising laminating a non-birefringent film to the planar retarderfilm, the planar retarder film being laminated between thenon-birefringent film and the polarization film.
 8. An optical filter ina curved configuration, the filter comprising: a retarder layer coupledto a polarizer layer; wherein the retarder layer and polarizer layer areconfigured to have a compound curvature, wherein the retarder layer ismade of a material having a strain optic coefficient K, a thickness ofd, and a retardance of Γ, and wherein the ratio$\frac{K \cdot d \cdot ɛ^{''}}{\Gamma}$ is less than 0.01, where “∈″” isthe magnitude of a strain tensor induced on the filter by the compoundcurvature.
 9. The filter of claim 8, wherein the retarder layer is madeof a material whose strain optic coefficient is between 0.001 to 0.025.10. The filter of claim 8, wherein the retarder layer has a thicknessbetween 50 to 120 micron.
 11. The filter of claim 8, wherein the opticalfilter was thermoformed.
 12. The filter of claim 8, wherein the retarderlayer is substantially made of cyclic olefin block copolymer.
 13. Thefilter of claim 8, the filter further comprising a substrate layercoupled to either the retarder layer of the polarizer layer.
 14. Thefilter of claim 8, wherein the coupling comprises lamination.
 15. Anoptical filter in a curved configuration, the filter comprising: apolarization layer; and a retarder layer laminated to the polarizationlayer at a predetermined angle, wherein the retarder and polarizationlayers are bent to the curved configuration, wherein the retarder layerhas a stress-induced birefringence, and further wherein thepredetermined angle substantially compensates for the stress-inducedbirefringence and disposes the optical axis of the retarder film in adesired orientation.
 16. The filter of claim 15, wherein an outersurface of the polarizer layer is laminated to an inner surface of theretarder layer.
 17. The filter of claim 16, further comprising anon-birefringent layer laminated to an outer surface of the retarderlayer, the non-birefringent layer being operable to compensate thestress birefringence in the retarder layer.
 18. The filter of claim 15,wherein an outer surface of the retarder layer is laminated to an innersurface of the polarizer layer.
 19. The filter of claim 18, furthercomprising a non-birefringent layer laminated to an inner surface of theretarder layer, the non-birefringent layer being operable to compensatethe stress birefringence in the retarder layer.
 20. The filter of claim15, wherein the polarizer and retarder layers are laminated with apressure sensitive adhesive.
 21. The filter of claim 15, wherein theretarder layer comprises a multi-layer retarder stack.
 22. The filter ofclaim 15, wherein the retarder layer comprises a quarter wave plate. 23.The filter of claim 15, wherein the planar retarder film is made ofcyclic olefin block copolymer or cellulose di-acetate.